1. Field of the Invention
The invention is drawn to recombinant microorganisms effective for producing xylitol from arabinose.
2. Description of the Prior Art
Xylitol is a non-sugar sweetener approved for use in foods and other items in many countries. It has the same order of sweetness as sucrose with fewer calories and no unpleasant aftertaste. It is also well-known that xylitol is an anticariogenic sweetener. These properties make xylitol an extremely useful food ingredient.
Microbial production of xylitol, using hemicellulosic biomass such as agricultural residues, is attractive for reducing its manufacturing cost. The hemicellulose hydrolyzates are generally rich in D-xylose and L-arabinose (Saha, 2003, J. Ind. Microbiol. Biotechnol. 30:279-291). D-xylose can be metabolized in xylose-fermenting yeasts such as Candida, Debaromyces, and Pichia species. These yeasts have xylose reductase, which reduces D-xylose to xylitol. Xylitol production from D-xylose has been well studied using the xylose-fermenting yeasts and their reductases [Saha & Bothast, Microbial production of xylitol. In: Saha, B. C. and Woodward, J. (eds.) Fuels and Chemicals, ACS Symposium Series 666, American Chemical Society, pp. 307-319, 1997]. However, in these systems, the arabinose is a particular problem to xylitol production because it can be easily converted to arabitol, which is very difficult to separate from the xylitol in a cost efficient manner.
In contrast to xylose, no xylitol production process from L-arabinose has been developed, despite its high abundance in the hemicellulosic hydrolyzates. Certain species of fungi have been reported to possess an L-arabinose catabolic pathway that synthesizes xylitol as an intermediate (Chang & Knight, 1960, Biochem. Biophys. Res. Commun. 3:554-559). However, no bacteria have been reported for producing xylitol from arabinose. Thus, the need remains for an improved fermentative process for producing xylitol from arabinose.